Novel Organic-Inorganic Composite Microgels And Their Selective Adsorption To Heavy Metal Ions

Heavy metal pollutions are serious issues in the areas of mining and deserted mine sites. Mining activities often cause water, atmosphere and subsequently soil pollution. Different from organic compounds, heavy metal pollutions are non-biodegradable and could accumulate in living organisms. Consequently, all living organisms within a given ecosystem could be affected through the food chain. As such, plants adsorb heavy metals in polluted soils and gain a certain accumulation in their tissues; animals feeding on the plants or polluted waters would also have these metals accumulated in their tissues; humans are in turn exposed to heavy metals by consuming contaminated plants and animals. Research has verified that some of heavy metals may cause a range of ailments and seriously threaten ecosystem and public health. Water is susceptible to heavy metal pollution through industrial wastewater disposals. The fluidity and permeability of water often spread the pollutants easily, which results in much severer and more uncontrollable damages to environment and all living organisms. Therefore, the water contamination from heavy metals has been great concerns in many environmental considerations in recent years.At present, heavy metal-contained wastewater cannot be disposed to the nature without adequate treatment. Among many methods for separating and removing heavy metal ions from water, which include chemical precipitation, physical adsorption, chemical adsorption, ion exchange, extraction and biological treatment, chemical precipitation and physical adsorption are not very effective for treating wastewater containing heavy metal at low concentration; ion exchange and extraction have the problem of secondary contamination; and biological treatment is not widely applicable; on the other hand, chemical adsorption has exhibited many advantages, such as reaching adsorption condition easily, broad range adaptability, easy to deploy, and excellent adsorption effectiveness. As a result, the chemical adsorption process is currently one of the major techniques for the removal of heavy metal ions form wastewater.Some natural high molecular materials, such as sawdust, waste eggshell, blue green algae (cyanobacteria), are often used as chemical adsorbents to remove heavy metals. They not only show some degree of adsorption to heavy metals, but also have the advantages of environmentally friendly biodegradability and low cost since they are either abundant in nature, or by-product or waste material from industry. However, with biodegradation, these materials produce micromolecules which are soluble in water, thus cause the secondary water pollution. Also due to the bio-degradation, the reusability of these materials are poor as well. There also have been reports on clays used to remove heavy metals, such as bentonite, attapulgite, zeolite. True value of these materials is their low cost, while the problems for using these materials include poor reusability due to the difficulties of separating them from water and excessive pH-susceptibility. In recent years, polymer adsorbents and polymer/inorganic nanocomposite adsorbents have become hot research topics due to their excellent stability, good adsorption capacity and good adsorption selectivity to heavy metal ion.In this study, we attempted to address the above-mentioned problems by using inexpensive, functionalized clay or waste material from industry instead of cross-linking agents to prepare novel organic-inorganic hybrid composite adsorbents.1. A facile strategy was developed to synthesize the novel attapulgite/poly(acrylic acid)(ATP/PAA) nanocomposite microgels via the "one-pot" inverse suspension polymerization with the multifunctionalized attapulgite (ATP) as the unique cross-linker. The multifunctionalized ATP was in situ produced directly in the form of water-in-oil emulsion and then used as prepared for the inverse suspension polymerization without being separated; so the proposed strategy was much simpler compared with the traditional methods. Almost all AA monomers had been successfully grafted onto ATP to form the3-dimensional cross-linking network of the nanocomposite hydrogels, in which the multifunctionalized ATP nanorods, as cross-linker and structural strengthening agent, drastically improved the mechanical stability of the resulting ATP/PAA hydrogels. The nanocomposite hydrogels exhibited selective adsorption toward the Pb2+ion with a maximum adsorption capacity of42mg/g, and the adsorbed Pb2+ion could be eluted completely. The optimized nanocomposite hydrogel adsorbent also exhibited an excellent reusability. The adsorption capacity and desorption rate remained unchanged for at least10cycles of adsorption-desorption. These strong points make it a potential adsorbent for heavy metal-contaminated water.2. Novel covalently crosslinked microbeads of the attapulgite/poly(acrylic acid-co-acrylamide)(ATP/P(AA-co-AM)) hybrid hydrogels with excellent mechanical stability were synthesized via inverse suspension copolymerization of acrylic acid (AA) and acrylamide (AM) with the multifunctional ATP nanorods as sole crosslinker. The synthesis conditions, such as feeding method, neutralization degree of AA, and feeding ratio of these comonomers to the multifunctional ATP, were optimized scientifically. The TGA result showed that near100%comonomers had been grafted onto the multifunctional ATP nanorods to form the3-dimensional network skeleton of the hybrid hydrogels, with the feeding ratio of the multifunctional ATP nanorods and the comonomers of1:5. The ATP/P(AA-co-AM) hybrid nanocomposite microgels exhibited the selective adsorption toward the toxic heavy metal ions, especially for Pb2+and Cu2+ions, and the adsorbed ions could be easily desorbed, indicating the reusability of the ATP/P(AA-co-AM) hybrid hydrogels. Their use as an adsorbent for the toxic heavy metal ions can therefore be expected to be economically and technically feasible.3. Novel magnetic fly ash/poly(acrylic acid)(FA/PAA) composite microgel and fly ash/poly(acrylic acid-co-acrylamide)(FA/P(AA-co-AM)) composite microgel were respectively designed as selective adsorbents towards Pb2+based on the " treating wastewater with wastes " strategy. The magnetic fly ash (FA) was selected as inorganic crosslinker to produce the magnetic composite microgels under an inverse suspension polymerization process, after being surface-modified with the polymerizable groups. The monomers had been successfully grafted onto the magnetic FA to form a3-dimentional crosslinked beadlike magnetic composite microgels without any foreign organic crosslinker added. Both microgels possessed high adsorption capacity and good adsorption selectivity to Pb2+owing to the carboxyl groups and the amide groups respectively form PAA and P(AA-co-AM). The introduction of the magnetic FA could not only improve the strength stability of the composit microgels, but also provide them the ability of magnetic separation. Furthermore, the adsorbed Pb2+could be easily desorbed, indicating the reusability of the beadlike magnetic composite microgels.4. The functionalized ATP (an inexpensive and locally available clay) and the FA (a by-product from coal-burning power plants) were used as cross-linkers to prepare the nanocomposite magnetic beads by the inverse suspension polymerization process. The results from FT-IR and TGA revealed that the soft and elastic PAA blocks and P(AA-co-AM) blocks were separately grafted onto the rigid3-D inorganic skeletons of ATP and FA successfully to produce fly ash/poly(acrylic acid)/attapulgite (FA/PAA/ATP) nanocomposite magnetic beads and fly ash/poly(acrylic acid-co-acrylamide)/attapulgite (FA/P(AA-co-AM)/ATP). Both nanocomposites exhibited outstanding mechanical strength due to the large aspect ratio of ATP, a certain magnetic characteristic provided by the substantial Fe3O4in FA, good adsorption capacity to heavy metal ions, especially to Pb2+, and a strong prospect for further industrialization because of the decrease in material cost. The adsorption capacity and the desorption rate remained almost unchanged within the limit of error through5cycles of regeneration, indicating that both nanocomposite microgels have excellent reusability and can be used in the continuous process of wastewater treatment in further industrialization, on the other hand, lower the production cost further.